A gas turbine engine centrifugal compressor includes a rotating impeller arranged to accelerate and, thereby, increase the kinetic energy of air flowing therethrough. A diffuser is generally located immediately downstream of and surrounding the impeller. The diffuser operates to decrease the velocity of the air flow leaving the impeller and transform the energy thereof to an increase in static pressure, thus, pressurizing the air.
Diffusers have generally included a plurality of circumferentially spaced passages which converge to an annular space surrounding the impeller. These passages expand in area downstream of the impeller in order to diffuse the flow exiting the impeller. One such diffuser is disclosed in U.S. Pat. No. 4,027,997 issued to A. C. Bryans on Jun. 7, 1977, and assigned to the assignee of this patent. The diffuser passages in this patent assume an initial circular cross section so as to accommodate with minimal losses the relatively high-flow velocities of the air exiting the impeller and, thereafter, gradually merge into a near-rectangular outlet to minimize losses. Each passage gradually merges from a circular cross section at a throat portion near its inlet end, to a near rectangular cross section at its outlet end defined by two flat opposing parallel sides and two flat opposing curved sides which produce a razor sharp trailing edge at the diffuser outlet. This near rectangular shape of the diffuser outlet optimizes the flow distribution to an annular combustion chamber in flow communication with the diffuser outlet.
A diffuser in U.S. Pat. No. 4,576,550 issued to A. C. Bryans on Mar. 18, 1986, and assigned to the assignee of this patent discloses each of the passages includes a throat portion having a quadrilateral cross section, including two substantially parallel linear sidewalls and two substantially arcuate opposing sidewalls, effective for reducing the length of and, thereby, pressure losses from the annular inlet. The linearity and regularity of the diffuser passages enables the diffuser to be manufactured to close tolerances by electric discharge milling an annular plate utilizing a single tool. This assures uniformity and consistency between diffusers. U.S. Pat. No. 4,576,550 is incorporated herein by reference.
We have found that these diffuser designs either reduce trailing edge blockage with greater than optimum area ratios or with large trailing edge blockages that impair performance of downstream components that remove swirl before flow enters the combustor.
Thus, there continues to be a demand for advancements in diffuser design and geometry that improves aerodynamic performance and reduces the overall engine radial envelope.